Achieving Stable Earth Leakage Performance in VFD Based Mining Operations

Guide for Mining Engineers and MCC Specialists responsible for VFD driven motors and long cable installations in harsh mining environments. 

This guide explains how to coordinate earth leakage (EL) protection in Variable Frequency Drive (VFD) systems to reduce nuisance trips and improve uptime. The guidance is indicative and must always be verified against your site standards and commissioning procedures.

Quick Tips:

• Use selective measured settings and VFD Capable EL devices.
• Measure each feeder’s steady-state and start-transient leakage and set per-feeder EL thresholds and time delays to provide selectivity with upstream relays.
• Allow for VFD-induced leakage and the effect of long cable capacitance.
• Zone EL and verify upstream power quality (PQ) to prevent false trips.

VFDs are widely used in mining MCCs for conveyors, crushers, and slurry pumps because they provide precise motor control and soft starts. However, they inherently produce high-frequency leakage currents through the motor cable and to earth. The switching frequency of the drive and the total cable capacitance (especially in long runs) contribute significantly to leakage.

• VFD filters: Installing output filters or reactors reduces leakage, but does not eliminate it.
• Cumulative effect: When several drives share a common EL device, the combined leakage can exceed trip thresholds.

Tip: When multiple VFDs are present, consider per-feeder EL detection and relays with VFD-cabable detection and programmable trip curves use a relay like the MA Relay (BBRTU).  Discrimination still requires correct thresholds, CT Placement and commissioning tests.

Setting EL thresholds and time delays without sacrificing safety

Mining environments require sensitive earth leakage protection due to the high risk of shock and fire. Yet, if the EL trip level is set too low, harmless leakage from the VFD or long cables can trigger unwanted shutdowns.

Best practice guidelines (indicative):

• Trip threshold: Start from 100–300 mA for individual VFD feeders, depending on drive size and cable length.
• Time delay: choose a delay sufficient to ride through known transient leakage during starts; typical coordination delays are tens to a few hundred milliseconds, but final settings must be verified by injection/startup tests and be coordinated with upstream protection.
• Ramp coordination: Increase delay progressively for higher-rated circuits to achieve selectivity.
• Testing: Always verify with injection testing and simulated load starts before commissioning.

Use Type B earth leakage relays or VFD-compatible designs that can detect DC and high-frequency components. Devices like NewElec’s MA Relay are engineered for differential protection with programmable trip curves, allowing site engineers to fine-tune EL performance.

Try the Relay Selector to pre-filter protection options by current range, mounting, and EL compatibility.

Coordinating EL with motor relays and MCC zoning

Proper zoning is crucial to prevent the entire MCC from tripping when only one motor feeder experiences a fault. Mining MCCs often contain 10–30 outgoing circuits feeding crushers, conveyors, or pumps. If all share a single upstream EL, minor leakage can cascade into a full shutdown.

Coordination guidelines:

1. Assign individual EL relays per motor circuit or per zone.
2. Use differential protection within each zone, with a higher threshold upstream.
3. Integrate EL feedback into motor protection relays (like the MA Relay) for unified fault reporting.
4. Ensure thermal protection and start supervision functions remain unaffected by EL delay settings.

By coordinating EL with the motor relay’s internal logic, engineers can achieve selective tripping: the affected feeder isolates, while upstream zones remain stable.

 See start supervision and thermal coordination features in the MA Relay Deep Dive.

Managing long cable runs and EMI for Mining MCCs

Mining installations often involve long motor cable runs, sometimes exceeding 100–200 meters between the MCC and field equipment like conveyors or slurry pumps. These lengths amplify both capacitive leakage and electromagnetic interference (EMI).

Engineering considerations:

• Cable type: Use screened or armoured cables with symmetrical earth returns.
• EMC filters: Fit common-mode chokes or sine filters on the VFD output to minimize HF noise.
• Grounding: Ensure multiple low-resistance earth points to disperse high-frequency leakage safely.
• Segregation: Route signal and control cables separately to reduce induced noise in instrumentation.

EMI can mimic EL faults, particularly if shielding is terminated incorrectly or cable trays share noisy circuits. Mining sites that operate under variable loads, wet conditions, and conductive dust are especially vulnerable.

Verifying power quality upstream to prevent cascading trips

Even a perfectly tuned EL system can experience nuisance tripping if upstream power quality is unstable. Voltage dips, harmonics, or phase imbalance can cause spurious current flow to earth or drive internal protection to activate incorrectly.

PQ checks to include during commissioning:

• Voltage balance: ≤ 2% imbalance between phases.
• Harmonics: Maintain total harmonic distortion (THD) below 5%.
• Neutral earthing: Inspect for stray currents or shared neutrals between VFD panels.
• Feeder protection: Use relays capable of PQ monitoring, such as NewElec’s KD Feeder Relay range.

Stable PQ ensures accurate EL operation and extends the life of drives, relays, and motors. For advanced coordination, the PQ data can also feed into predictive maintenance dashboards, reducing unplanned stoppages.

Field commissioning checklist and common pitfalls

Use this indicative checklist to streamline commissioning and reduce startup troubleshooting:

1. Verify cable lengths and insulation resistance for each circuit.
2. Confirm VFD filters, reactors, or output chokes are installed as designed.
3. Set EL thresholds 50% above the measured steady-state leakage current.
4. Calibrate time delays progressively by MCC zone.
5. Test the EL relay operation using current injection or portable testers.
6. Validate the correct direction of CT installation (for differential EL relays).
7. Check PQ stability during simultaneous conveyor or crusher startup.
8. Record baseline leakage currents for future maintenance comparison.

Common pitfalls:

• Shared neutrals or earthing between panels.
• Oversensitive EL relays with default factory settings.
• Ignoring drive-specific leakage data from the manufacturer.
• Skipping re-tests after cable or motor replacement.

Following a structured commissioning plan avoids many of the nuisance trips that plague complex MCCs in mining plants.

For step by step examples, refer to the dedicated Q&A guide “How To Coordinate Earth Leakage Protection for VFDs in Mining MCCs” in the Knowledge Hub. The Q&A walks through two applied field scenarios, including a 90 kW conveyor VFD with a 150 m cable run and a 250 kW crusher on a soft starter with long feeds and multiple earth points.

In that guide you can see how measured leakage values, time graded settings, and thermal supervision are used to choose indicative trip thresholds and delays, and how these settings prevent nuisance trips while keeping personnel and equipment protected.